Dispensing devices such as application heads, are utilized in industry, for example, in order to coat areas of film or foil substrates with liquid adhesive such as hot melt adhesive. The fluid material flows out of a source of material, normally a reservoir, into the flow channel of the device, passes through a valve body, and continues to flow to a nozzle arrangement with an outlet opening. Frequently a so-called intermittent application is performed, meaning that intervals in which the valve body is in the open position and material is applied to the substrate alternate with intervals in which the valve body is in the closed position, so that the application of material is interrupted. Often, very short intervals are used in intermittent applications in order to realize application zones at very small distances from each other.
The application pattern that is produced on the substrate is normally subject to the requirement that a material application zone on the substrate have sharply delimited edges. In the case of a large-area application with the help of a known slit nozzle arrangement, it is especially desired that not only the lateral edges (in the direction of motion of the substrate relative to the application device) but also the front and rear edges of a material application zone be sharply delimited. A prerequisite for such sharp delimitation of the front and rear edges is that the valve body of the valve arrangement is moved quickly into its closed position, so that the flow of material from the outlet opening is interrupted uniformly quickly. When the valve arrangement is opened, in order to attain a sharp boundary line at the front edge of a material application zone it is necessary for the valve arrangement to open quickly and for the application of material to begin without delay.
A needle valve has been used for this purpose, having a needle with a needle tip as a valve body, which may be brought into contact with a valve seat that conforms to the shape of the needle tip. To close the valve arrangement, the needle (under electro-pneumatic actuation) is moved in the direction of the valve seat and comes into contact with the latter, so that the flow cross section of the flow channel is closed and the flow of material is thereby interrupted. During the closing motion of the needle tip, some adhesive is moved downstream by the needle tip in the direction of the outlet opening. As a result, the application of material to the substrate is not interrupted as abruptly as would be necessary to produce a sharp boundary line in the end area of an application zone. An “afterdrip” from the outlet opening during closure of the valve arrangement cannot be prevented.
A reduction of such an afterdrip of material from the outlet opening was achieved by an application head known from the published patent EP-A-0 850 697, in which a valve body that is enlarged compared to a valve shaft is moved upstream to close the valve arrangement, i.e., counter to the direction of flow of the material in the open position in the direction of the outlet opening of a nozzle arrangement. The result of this arrangement is that during the closing motion of the valve body, because of adhesion of the material to the enlarged valve body, and because of material being drawn along, there is a slight backflow of material upstream. A relatively abrupt interruption of the flow of material from the outlet opening results and it is largely possible to prevent afterdripping.
The object of the present invention is to further improve intermittent dispensing devices such that the flow of material out of an outlet opening is interrupted more abruptly, and in particular afterdripping may be prevented even more effectively resulting in very sharply delimited material application zones or application patterns on a substrate.